Chemistry: Ion Size and Radius Effects on Ionic Bonds

Ion Size and Radius Effects

• Ion size and radius affect the strength of ionic bonds:
  • Smaller ions have a higher charge density, resulting in stronger bonds
  • Larger ions have a lower charge density, resulting in weaker bonds
• Radius ratio rule (see below) helps predict the type of lattice structure formed based on ion size and radius

Lattice Energy

• Lattice energy: the energy required to separate one mole of solid ionic compound into its constituent ions in the gas phase
• Lattice energy is directly proportional to the charge of the ions and inversely proportional to the distance between them
• Higher lattice energy indicates a stronger ionic bond

Radius Ratio Rule

• Predicts the type of lattice structure formed based on the radius ratio of the cation to the anion
• Radius ratio = r(cation) / r(anion)
• Radius ratio ranges:
  • <0.15: linear or bent molecules
  • 0.15-0.41: trigonal planar
  • 0.41-0.73: tetrahedral
  • 0.73-1.37: octahedral
  • >1.37: cubic or body-centered cubic

Madelung Constant and Born-Haber Cycle

• Madelung constant: a factor that accounts for the electrostatic interactions between ions in a lattice
• Born-Haber cycle: a thermodynamic cycle that calculates the lattice energy of an ionic compound
• Born-Haber cycle involves:
  1. Atomization of elements to form gaseous atoms
  2. Ionization of gaseous atoms to form ions
  3. Formation of the ionic lattice from the ions
• Lattice energy is calculated from the sum of the energies involved in these steps

Solubility and Dissolution Energy

• Solubility of an ionic compound is influenced by the lattice energy and the hydration energy of the ions
• Dissolution energy: the energy required to dissolve one mole of an ionic compound in water
• Dissolution energy is the sum of the lattice energy and the hydration energy
• Higher lattice energy and lower hydration energy result in lower solubility

Ion Size and Radius Effects

• Smaller ions have a higher charge density, resulting in stronger ionic bonds
• Larger ions have a lower charge density, resulting in weaker ionic bonds
• Radius ratio rule helps predict the type of lattice structure formed based on ion size and radius

Lattice Energy

• Lattice energy is the energy required to separate one mole of solid ionic compound into its constituent ions in the gas phase
• Lattice energy is directly proportional to the charge of the ions and inversely proportional to the distance between them
• Higher lattice energy indicates a stronger ionic bond

Radius Ratio Rule

• Radius ratio = r(cation) / r(anion)
• Radius ratio ranges determine the type of lattice structure formed
• For example, a radius ratio of 1.37 results in a cubic or body-centered cubic lattice structure

Madelung Constant and Born-Haber Cycle

• Madelung constant accounts for the electrostatic interactions between ions in a lattice
• Born-Haber cycle calculates the lattice energy of an ionic compound
• Born-Haber cycle involves three steps: atomization, ionization, and lattice formation
• Lattice energy is the sum of the energies involved in these three steps

Solubility and Dissolution Energy

• Solubility of an ionic compound is influenced by lattice energy and hydration energy
• Dissolution energy is the energy required to dissolve one mole of an ionic compound in water
• Dissolution energy is the sum of lattice energy and hydration energy
• Higher lattice energy and lower hydration energy result in lower solubility

Explore how ion size and radius impact the strength of ionic bonds, and learn about the radius ratio rule and lattice energy in ionic compounds.